If your nanny is in charge, let her do things the way you both wanted. Your mother is simply a visiting guest & unless you authorize her to do so, she really shouldn't be interfering w/the nannies duties.
I can understand you feeling caught up in the middle of things, but you hired your nanny for a reason so let her perform her duties as you are paying her to do.
I understand your mom's point of view, after all, these are her precious grandchildren. But she needs to respect your nanny's boundaries & not interfere. Esp. if she is leaving in a week.
After she leaves, then you + your nanny can sit down and work between yourselves a plan that you can both agree as well as adhere to.
Good luck to both of you.